This application relates to imaging devices and techniques, and more particularly to imaging devices and techniques based on semiconductor sensors.
Image sensors are widely used in many applications to produce images of various objects. Imaging circuits often include a two-dimensional array of photosensors that are designed to produce output signals in response to photons. Each photosensor can be used to form a portion or the entirety of one picture element (pixel) of the image. The individual photosensors can be scanned to read out and process the output signals for various imaging operations.
One class of solid-state image sensors includes an array of active pixel sensors (APS) formed on a semiconductor substrate. An APS is a light sensing device with sensing circuitry inside each pixel. Each active pixel includes a sensing element formed in a semiconductor substrate and capable of converting optical signals into electronic signals. As photons strike the surface of a photoactive region within each active pixel, free charge carriers are generated and collected. Once collected, the charge carriers are converted into an electrical signal within each pixel. Hence, distinctly different from a charge coupled device (CCD) or a metal oxide semiconductor (MOS) diode array, an APS device does not transfer charge from one pixel to another for readout. The APS can convert the photocharge to an electronic signal prior to transferring the signal to a common conductor that conducts the signals to an output node.
APS devices can be fabricated in a manner compatible with complementary metal oxide semiconductor (CMOS) processes. Compatibility with CMOS processes allows many signal processing functions and operation controls to be integrated on an APS chip at a relatively low cost. CMOS circuitry also allows simple power supplies to be used and can result in reduced power consumption. Moreover, the active pixels of APS devices allow non-destructive readout, simplified digital interface, and random access.